When global temperature changes over tens of years are considered, so that local and more temporary changes are averaged out, there is no doubt that global warming is happening.

It is true that the amount of carbon dioxide in the atmosphere has been rising slowly for the last 5,000 years, but this has been in response to the natural cycle driven by variations in the Earth’s orbit around the Sun. Superimposed on this are natural and local changes in climate caused for example by violent volcanic eruptions and by changes to current patterns in the oceans. However, following the industrial revolution in the 1700s, the concentration of carbon dioxide has increased very steeply. According to the latest Intergovernmental Panel on Climate Change report (2007), it is more than 90% certain that the world is already warming as a result of human activity.

The ‘Little Ice age’ of 17th -19th century Europe was not a global change, but was regional. The 20th century changes in surface temperature, by contrast, have been global. The global cooling from 1940 -1970 was caused by the injection of small particulates and aerosols into the atmosphere by polluting industries, now largely stopped by clean air legislation. Sulphur dioxide emitted during volcanic eruptions and a reduction in solar activity also contributed to this cooling.

The apparent global cooling since 1998 only appears when temperatures are averaged over less than ten years; the long-term (1998-2007) trend is still upwards. This is even clearer when a short-term warming caused by ocean currents in 1998, the El Niño effect, is removed.

The planet’s atmosphere is a chaotic system, with many complex and interconnected effects, which will cause some years to be warmer or cooler than others. So it is important to look at the trends over tens of years or longer. Some authors have examined temperature over unduly short periods of time, leading to a mistaken interpretation of the long term global trend, which is undeniably that of increasing temperature.